Air Quality

Air quality is important for human health, vegetation, and crops as well as aesthetic considerations (such as visibility) that affect appreciation of the natural beauty of national parks and other outdoor spaces. Many of the processes that determine air quality are affected by weather (Figure 13.1). For example, hot, sunny days can increase ozone levels, while stagnant weather conditions can produce high concentrations of both ozone and particulate matter (PM). Ozone and PM are air pollutants that adversely affect human health and are monitored and regulated with national standards. Temperature, wind patterns, cloud cover, and precipitation, as well as the amounts and types of pollutants emitted into the air from human activities and natural sources, all affect air quality (Figure 13.1). Thus, climate-driven changes in weather, human activity, and natural emissions are all expected to impact future air quality across the United States.

These climate effects on air quality are not expected to occur uniformly at all locations. For example, as discussed in Chapter 2: Climate, precipitation is projected to increase in some regions of the country and decrease in other regions. Regions that experience excessive periods of drought and higher temperatures will have increased frequency of wildfires and more windblown dust from soils. At the same time, changes to temperatures and rainfall affect the types of crops that can be grown (Ch. 10: Ag & Rural) and the length of the growing season, the application of fertilizers and pesticides to crops, and ensuing transport and fate of those chemicals into the air, water, and soil. In the future, climate change is expected to alter the demand for heating and cooling of indoor spaces due to changes in temperatures. The resulting shift in fuel types and amounts used will modify the amount and composition of air pollutants emitted. Climate change can also increase the duration of the pollen season and the amount of pollen at some locations, as well as worsen respiratory health impacts due to pollen exposure. Despite the potential variability in regional impacts of climate change, there is evidence that climate change will increase the risk of unhealthy air quality in the future across the Nation in the absence of further air pollution control efforts (for other impacts of climate change on health, see Ch. 14: Human Health).

Since people spend most of their time inside buildings, indoor air quality is important for human health. Indoor air pollutants may come from interior sources or may be transported into buildings with outdoor air. If there are changes in airborne pollutants of outdoor origin, such as ozone, pollen, mold, and PM2.5 (particulate matter less than 2.5 micrometers in diameter), there will be changes in indoor exposures to these contaminants.2,3

There is robust evidence from models and observations that climate change is worsening ozone pollution. The net effect of climate change on PM pollution is less certain than for ozone, but increases in smoke from wildfires and windblown dust from regions affected by drought are expected. The complex interactions of natural variability with changes in climate and emissions pose a significant challenge for air quality management. Some approaches to mitigating climate change could result in large near-term co-benefits for air quality.

Figure 13.1: Pathways by Which Climate Change Will Influence Air Pollution

Air Pollution Health Effects

Ground-level ozone and particulate matter are common air pollutants that pose a serious risk to human health and the environment.5,6 Short- and long-term exposure to these pollutants results in adverse respiratory and cardiovascular effects,7 including premature deaths,8 hospital and emergency room visits, aggravated asthma,3,9 and shortness of breath.10 Certain population groups, such as the elderly, children, and those with chronic illnesses, are especially susceptible to ozone and PM-related effects.11,12,13

A growing body of evidence indicates the harmful effects of short-term (i.e., daily) exposures to ground-level ozone vary with climate conditions, specifically temperature.14,15,16,17,18 For a given level of ozone, higher temperatures increase the risk of ozone-related premature death.14,19,20,21 However, the risk of premature death is likely to decrease as the prevalence of air conditioning increases, as is expected to occur with rising temperatures.22 The extent to which the growing use of air conditioning will offset climate-induced increases in ozone-related premature death is unknown.

Ozone Air Quality

Ozone is not directly emitted but is formed in the atmosphere by reactions between nitrogen oxides (NOx) and volatile organic compounds (VOCs). Ozone concentrations depend on emissions of these two precursors as well as weather conditions such as temperature, humidity, cloud cover, and winds.3 These emissions come from a variety of human sources, such as power plants and motor vehicles, and from natural sources, such as forests and wildfires (Figure 13.1). Additionally, ozone concentrations in one region may be influenced by the transport of either precursors or ozone itself from another region.23,24

Ozone levels in the United States are often highest in Southern California and the Northeast Corridor as well as around other large cities like Dallas, Houston, Denver, Phoenix, and Chicago,25 and during extended episodes of extreme heat and sunshine.26 Ozone air quality in the United States has improved dramatically over the past few decades due to NOx and VOC emissions control efforts, despite population and economic growth.27,28,29 Nationally, ozone concentrations have been reduced by 22% over the 1990 to 2016 period.29 Nonetheless, in 2015 nearly 1 in 3 Americans were exposed to ozone values that exceeded the national standard determined by the U.S. Environmental Protection Agency (EPA) to be protective of human health.29 Adverse human health impacts associated with exposure to ground-level ozone include premature death, respiratory hospital admissions, cases of aggravated asthma, lost days of school, and reduced productivity among outdoor workers.30,31,32 Ozone pollution can also damage crops and plant communities, including forests, by reducing photosynthesis.33

Due in part to air pollutant regulations driven by the Clean Air Act, NOx and VOC emissions from human sources should continue to decline over the next few decades.34 These emissions reductions are designed to reduce ozone concentrations so that polluted areas of the country meet air quality standards. However, climate change will also influence future levels of ozone in the United States by altering weather conditions and impacting emissions from human and natural sources. The prevailing evidence strongly suggests that climate change alone introduces a climate penalty (an increase in air pollution resulting from climate change35,36) for ozone over most of the United States from warmer temperatures and increases in natural emissions.3,4,37,38 This climate penalty will partially counteract the continued reductions in emissions of ozone precursors from human activities.

Particulate Matter

Tiny liquid or solid particles suspended in the atmosphere are known as aerosols or particulate matter (PM). PM includes many different chemical components, such as sulfate, nitrate, organic and black carbon, mineral dust, and sea spray. Unlike ozone, PM can be either directly emitted or formed in the atmosphere. PM2.5 refers to atmospheric PM with a diameter less than 2.5 micrometers. These particles are small enough to be inhaled deeply, and exposure to high concentrations can result in serious health impacts, including premature death, nonfatal heart attacks, and adverse birth outcomes.5,39,40,41 PM2.5 concentrations vary greatly with daily weather conditions,42,43 depending particularly on wind speed (which affects the mixing of pollutants) and precipitation (which removes particles from the air).4 Concentrations of PM2.5 build up during long periods of low wind speeds, and they are reduced when weather fronts move air through a region.4

Wildfires not only emit gases that contribute to ozone formation44,45,46,47,48 but they also are a major source of PM, especially in the western United States during the summer49,50,51,52,53,54,55 and in the Southeast48,56(see Ch. 6: Forests; Ch. 19: Southeast, Case Study “Prescribed Fire”; Ch. 24: Northwest; and Ch. 25: Southwest). Wildfire smoke can worsen air quality locally,57 with substantial public health impacts in regions with large populations near heavily forested areas.56,58,59,60,61 Exposure to wildfire smoke increases the incidence of respiratory illnesses, including asthma, chronic obstructive pulmonary disease, bronchitis, and pneumonia.62 Smoke can decrease visibility63 and can be transported hundreds of miles downwind, often crossing national boundaries.54,64,65,66,67,68,69

Climate change is expected to impact atmospheric PM concentrations in numerous ways.38,70 Changing weather patterns, including increased stagnation,71,72 altered frequency of weather fronts,73,74 more frequent heavy rain events,43 changing emissions from vegetation75,76 and human sources,77 and increased evaporation of some aerosol components78 will all affect PM concentrations. In addition, more frequent and longer droughts would lengthen the wildfire season79,80,81 and result in larger wildfires82,83 and increased dust emissions in some areas.84 Projections of regional precipitation changes show considerable variation across models and thus remain highly uncertain.85 Accurately assessing how PM2.5 concentrations will respond to the changing climate is difficult due to these complex and highly spatially variable interactions.